[hydrogen] optimize switch with string clauses

src/x64/lithium-x64.h

Index: src/x64/lithium-x64.h
diff --git a/src/x64/lithium-x64.h b/src/x64/lithium-x64.h
index b542071b7ed1c6a95f1daf9a3db6a5c90cc1007d..bf07ae5e84c233f54fdff04bd804aee09540106e 100644
--- a/src/x64/lithium-x64.h
+++ b/src/x64/lithium-x64.h
@@ -640,6 +640,20 @@ class LIsObjectAndBranch: public LControlInstruction<1, 0> {
 };
 
 
+class LIsStringAndBranch: public LControlInstruction<1, 1> {
+ public:
+  explicit LIsStringAndBranch(LOperand* value, LOperand* temp) {
+    inputs_[0] = value;
+    temps_[0] = temp;
+  }
+
+  DECLARE_CONCRETE_INSTRUCTION(IsStringAndBranch, "is-string-and-branch")
+  DECLARE_HYDROGEN_ACCESSOR(IsStringAndBranch)
+
+  virtual void PrintDataTo(StringStream* stream);
+};
+
+
 class LIsSmiAndBranch: public LControlInstruction<1, 0> {
  public:
   explicit LIsSmiAndBranch(LOperand* value) {